Metazoa are multicellular, eukaryotic organisms in the biological kingdom Animalia

Reproduction and development

Sexual reproduction is nearly universal in animals, such as these dragonflies.

See also: Sexual reproduction § Animals, and Asexual reproduction § Examples in animals

Nearly all animals make use of some form of sexual reproduction.^[21] They produce haploid gametes by meiosis; the smaller, motile gametes are spermatozoa and the larger, non-motile gametes are ova.^[22] These fuse to form zygotes,^[23] which develop via mitosis into a hollow sphere, called a blastula. In sponges, blastula larvae swim to a new location, attach to the seabed, and develop into a new sponge.^[24] In most other groups, the blastula undergoes more complicated rearrangement.^[25] It first invaginates to form a gastrula with a digestive chamber and two separate germ layers, an external ectoderm and an internal endoderm.^[26] In most cases, a third germ layer, the mesoderm, also develops between them.^[27] These germ layers then differentiate to form tissues and organs.^[28]

Repeated instances of mating with a close relative during sexual reproduction generally leads to inbreeding depression within a population due to the increased prevalence of harmful recessive traits.^[29][30] Animals have evolved numerous mechanisms for avoiding close inbreeding.^[31]

Some animals are capable of asexual reproduction, which often results in a genetic clone of the parent. This may take place through fragmentation; budding, such as in Hydra and other cnidarians; or parthenogenesis, where fertile eggs are produced without mating, such as in aphids.^[32][33]

Ecology

Predators, such as this ultramarine flycatcher (Ficedula superciliaris), feed on other animals.

Animals are categorised into ecological groups depending on how they obtain or consume organic material, including carnivores, herbivores, omnivores, detritivores,^[34] and parasites.^[35] Interactions between animals form complex food webs. In carnivorous or omnivorous species, predation is a consumer-resource interaction where a predator feeds on another organism (called its prey).^[36] Selective pressures imposed on one another lead to an evolutionary arms race between predator and prey, resulting in various anti-predator adaptations.^[37][38] Almost all multicellular predators are animals.^[39] Some consumers use multiple methods; for example, in parasitoid wasps, the larvae feed on the hosts' living tissues, killing them in the process,^[40] but the adults primarily consume nectar from flowers.^[41] Other animals may have very specific feeding behaviours, such as hawksbill sea turtles primarily eating sponges.^[42]

Hydrothermal vent mussels and shrimps

Most animals rely on the biomass and energy produced by plants through photosynthesis. Herbivores eat plant material directly, while carnivores, and other animals on higher trophic levels typically acquire it indirectly by eating other animals. Animals oxidize carbohydrates, lipids, proteins, and other biomolecules to unlock the chemical energy of molecular oxygen,^[43] which allows the animal to grow and to sustain biological processes such as locomotion.^[44][45][46] Animals living close to hydrothermal vents and cold seeps on the dark sea floor consume organic matter of archaea and bacteria produced in these locations through chemosynthesis (by oxidizing inorganic compounds, such as hydrogen sulfide).^[47]

Animals originally evolved in the sea. Lineages of arthropods colonised land around the same time as land plants, probably between 510 and 471 million years ago during the Late Cambrian or Early Ordovician.^[48] Vertebrates such as the lobe-finned fish Tiktaalik started to move on to land in the late Devonian, about 375 million years ago.^[49][50] Animals occupy virtually all of earth's habitats and microhabitats, including salt water, hydrothermal vents, fresh water, hot springs, swamps, forests, pastures, deserts, air, and the interiors of animals, plants, fungi and rocks.^[51] Animals are however not particularly heat tolerant; very few of them can survive at constant temperatures above 50 °C (122 °F).^[52] Only very few species of animals (mostly nematodes) inhabit the most extreme cold deserts of continental Antarctica.^[53]